The climate of Mount Kenya has played a critical role in the development of the mountain, influencing the topography and ecology amongst other factors. It has a typical equatorial mountain climate which Hedberg described as winter every night and summer every day. Mount Kenya is home to one of the Global Atmosphere Watch's atmospheric monitoring stations.
The year is divided into two distinct wet seasons and two distinct dry seasons which mirror the wet and dry seasons in the Kenyan lowlands. As Mount Kenya ranges in height from 1,374 m (4,508 ft) to 5,199 m (17,057 ft) the climate varies considerably over the mountain and has different zones of influence. The lower, south eastern slopes are the wettest as the predominant weather system comes from the Indian ocean. This leads to very dense montane forest on these slopes. High on the mountain most of the precipitation falls as snow, but the most important water source is frost. Combined, these water sources feed 11 glaciers.
The current climate on Mount Kenya is wet, but drier than it has been in the past. The temperatures span a wide range, which diminishes with altitude. In the lower alpine zone they usually do not go below 12 °C (54 °F). Snow and rain are common from March to December, but especially in the two wet seasons. The wet seasons combined account for 5/6 of the annual precipitation. The monsoon, which controls the wet and dry seasons, means that most of the year there are south-easterly winds, but during January and February the dominate wind direction is north-easterly.
Mount Kenya, like most locations in the tropics, has two wet seasons and two dry seasons as a result of the monsoon. From mid-March to June the heavy rain season, known as the long rains, brings approximately half of the annual rainfall on the mountain. This is followed by the wetter of the two dry seasons which lasts until September. October to December are the short rains when the mountain receives approximately a third of its rainfall total. Finally from December to mid-March is the dry, dry season when the mountain experiences the least rain.
Mount Kenya straddles the equator. This means during the northern hemisphere summer the sun is to the north of the mountain. The altitude and aspect of the watersheds and main peaks results in the north side of the upper mountain being in summer condition. Simultaneously, the southern side is experiencing winter conditions. Once it is the southern hemisphere summer, the situation reverses.
During the dry season the mountain almost always follows the same daily weather pattern. Large daily temperature fluctuations occur which led Hedberg to exclaim winter every night and summer every day. There is variation in minimum and maximum temperatures day to day, but the standard deviation of the mean hourly pattern is small.
A typical day is clear and cool in the morning with low humidity. The mountain is in direct sunlight which causes the temperatures to rise quickly with the warmest temperatures occurring between 0900 and 1200. This corresponds to a maxima in the pressure, usually around 10am. Low on the mountain, between 2,400 m (7,874 ft) and 3,900 m (12,795 ft), clouds begin to form over the western forest zone, due to moist air from Lake Victoria. The anabatic winds caused by warm rising air gradually bring these clouds to the summit region in the afternoon. Around 1500 there is a minimum in sunlight and a maximum in humidity causing the actual and perceived temperature to drop. At 1600 there is a minimum in the pressure. This daily cover of cloud protects the glaciers on the south-west of the mountain which would otherwise get direct sun every day, enhancing their melt. The upwelling cloud eventually reaches the dry easterly air streams and dissipates, leading to a clear sky by 5pm. There is another maximum of temperature associated with this.
Being an equatorial mountain the day light hours are constant with twelve hour days. Sunrise is about 0530 with the sun setting at 1730. Over the course of the year there is a one minute difference between the shortest and longest days. At night, the sky is usually clear with katabatic winds blowing down the valleys. Above the lower alpine zone there is usually frost every night.